Charge storage devices containing carbon nanotube films as electrodes and charge collectors

ABSTRACT

An energy storage device includes a nanostructured network and an electrolyte in contact with the nanostructured network. The nanostructured network is an electrically conducting nanostructured network that provides combined functions of an electrode and a charge collector of the energy storage device. An electrical device includes an energy storage device that includes a nanostructured network and an electrolyte in contact with the nanostructured network, and a load-bearing electrical circuit electrically connected to the electrical energy storage device. The energy storage device is suitable to power the electrical device while in operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from, and is a 35 U.S.C. §111(a)continuation-in-part of, PCT international application serial numberPCT/US2008/004593, filed on Apr. 10, 2008, incorporated herein byreference in its entirety, which claims priority to U.S. provisionalapplication Ser. No. 60/907,587 filed on Apr. 10, 2007, incorporatedherein by reference in its entirety, and U.S. provisional applicationSer. No. 60/929,809 filed on Jul. 13, 2007, incorporated herein byreference in its entirety.

This application is also related to PCT International Publication No. WO2008/124167 published on Oct. 16, 2008, incorporated herein by referencein its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under 0454540, awardedby the National Science Foundation. The Government has certain rights inthe invention.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to charge storage devices suchas supercapacitors and batteries, and more particularly tonanostructure-films deposited on substrates for use in charge storageapplications.

2. Description of Related Art

The contents of all references, including articles, published patentapplications and patents referred to anywhere in this specification arehereby incorporated by reference.

Some charge storage devices such as supercapacitors and batteriesrequire high surface area materials that form a double layer with anelectrolyte. Different forms of carbonaceous materials such as carbonblack (M. Min et al J. Electrochem. Soc 153, A334 (2006), S-K. Kuo andL-L. Wu J. Power Sources 162,1437 (2006)) and carbon Nanotubes (CNT G.H. Deng et al Carbon 43, 1557 (2005), G-X Wang et al Solid State ionics176, 1169 (2005)) have been used as electrode materials. The highsurface area materials are used as the electrode, with the chargecollectors usually metals. Such configurations can have severaldisadvantages, especially for applications where flexibility, lightweight and thin films are required. Because of the metal chargecollectors, simple, room temperature fabrication processes are notapplicable. At the same time, cost and weight associated with the metalparts may be an issue.

As described herein, the term “electrode” refers to the portion of anenergy storage device that physically interacts with an electrolyte tostore charge (e.g., in a polarized double layer in a supercapacitor, ina chemical reaction in a battery, etc.). As described herein, the term“charge collector” refers to the electrically conductive element of anenergy storage device that is in substantial physical and electricalcontact with an electrode surface, and which serves to transfer chargefrom the electrode to an electrical contact pad (e.g., in an energystorage device wherein an electrode and a charge collector form thinfilms, each having essentially two planar surfaces, one of the electrodesurfaces is generally in physical contact with an electrolyte while theother electrode surface is in physical and electrical contact with acharge collector surface; the contacting electrode and charge collectorsurfaces generally share similar perimeter dimensions). Note: as usedherein, a charge collector can be distinguished from an electricalcontact pad in that an electrical contact pad has a small surfaceperimeter relative to that of a charge collector with which it is incontact (e.g., whereas the thin-film charge collector described abovehas a surface perimeter similar to that of the thin-film electrode withwhich it is in physical and electrical contact, the perimeter of acorresponding electrical contact pad connected to the charge collectorwill be much smaller (e.g., less than ˜40% of the surface perimeter ofthe charge collector).

Both batteries and supercapacitors are charge storage devices. Whilesupercapacitors store the electrical energy in polarized double layersalong the electrodes surfaces, batteries derive their energy fromchemical reactions in the active materials. A battery typically consistsof one or more cells which are in turn made up from two electrodes(anode and cathode) an electrolyte and a porous separator.

Batteries are classified into two groups, called primary and secondarybatteries. Primary batteries can only be used once and are notrechargeable. Because of their short lifespan the materials used inthese cells need to be cheap and environmental friendly. Several patents(U.S. Pat. No. 6,838,209 “Flexible thin battery and method ofmanufacturing same”; U.S. Pat. No. 6,858,349 “Battery cathode: carbonfibers”; U.S. Pat. No. 5,747,190 “Multilayered battery having a curedconductive ink layer”) describe such devices.

In many instances, carbon black and/or metals are being used as amaterial for current collectors and/or to increase the conductivity ofactive materials (MnO₂/carbon black paste) and as active electrodematerial (Li-ion batteries) in current battery designs. The disadvantageof using carbon black for these tasks is its relatively poor electricalconductivity (as compared to metals or carbon nanotubes), which requireslarge amounts of this material to be used. Metal current collectors,while providing good conductivity, have the disadvantage of having alarge mass. This, in turn, decreases the possible energy densities (perkg or per 1).

Batteries have, in addition to an electrolyte and separator, twocomponents: an anode and a cathode. As an example, MnO₂ together with anelectrically conducting medium such as carbon black is used as an anode,and for a cathode, a film of Zn is used. The configuration can not befabricated by a simple, solution based deposition process, the processrequired for the majority of applications.

There is thus a need for a charge storage device that is cheap, hasappropriate performance and can be disposed without creatingenvironmental hazards. An embodiment of the present invention includes adevice that can use carbon nanotubes (CNTs) to function as both anelectrode and a charge collector.

BRIEF SUMMARY OF THE INVENTION

An energy storage device according to an embodiment of the currentinvention includes a nanostructured network, and an electrolyte incontact with the nanostructured network. The nanostructured network isan electrically conducting nanostructured network that provides combinedfunctions of an electrode and a charge collector of the energy storagedevice. An electrical device according to an embodiment of the currentinvention includes an energy storage device according to an embodimentof the current invention and a load-bearing electrical circuitelectrically connected to the electrical energy storage device. Theenergy storage device is suitable to power the electrical device whilein operation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention can be better understood by reading the following detaileddescription with reference to the accompanying Figures in which:

FIG. 1 is a schematic illustration of a symmetric energy storage deviceutilizing nanowire or nanotube electrodes arranged in a percolatingnetwork according to an embodiment of the current invention. Theconfiguration shown represents a symmetric supercapacitor.

FIG. 2 is a schematic illustration of an asymmetric energy storagedevice utilizing nanowire or nanotube electrodes arranged in apercolating network according to an embodiment of the current invention.In addition, one electrode is coated with another material which becomesthe active material of that electrode. The configuration shownrepresents an asymmetric supercapacitor or a battery.

FIG. 3 is a schematic illustration of a symmetric energy storage deviceutilizing multiple networks (nanowire or nanotube network arranged in apercolating network coated with another material) as electrodesaccording to an embodiment of the current invention. The nanowire ornanotube network acts as current collector and the top-coated materialis the active material. The configuration shown represents a symmetricsupercapacitor.

FIG. 4 is a schematic illustration of an asymmetric energy storagedevice utilizing multiple networks (nanowire or nanotube networkarranged in a percolating network coated with another material that isdifferent on both sides) as electrodes according to an embodiment of thecurrent invention. The nanowire or nanotube network acts as a currentcollector and the top-coated material is the active material. Theconfiguration shown represents both, an asymmetric supercapacitor and abattery.

FIG. 5 is a schematic illustration of an asymmetric energy storagedevice utilizing multiple networks as electrodes, such as shown in FIG.4. The nanowire or nanotube network acts as a current collector and thetop-coated material is the active material. In order to avoid anunwanted reaction between current collector and active material of thesame electrode, an additional protective layer (e.g.poly(3,4-ethylenedioxythiophene) (PEDOT)) keeps the materials separatedbut electrically connected. The configuration shown represents both, anasymmetric supercapacitor and a battery.

FIG. 6 is a schematic illustration of a symmetric energy storage deviceutilizing composite electrodes (nanowires or nanotubes network mixedwith another material) according to an embodiment of the currentinvention. The configuration shown represents a symmetricsupercapacitor.

FIG. 7 is a schematic illustration of an asymmetric energy storagedevice utilizing composite electrodes (nanowires or nanotubes networkmixed with another material that is different for both sides) accordingto an embodiment of the current invention. The configuration shownrepresents an asymmetric supercapacitor or a battery.

FIG. 8 is a schematic illustration of an asymmetric energy storagedevice utilizing a metal sheet coated with an active material as oneelectrode and a multiple layer electrode (nanowires or nanotubesnetworks coated with another material, shown here as an example) or acomposite electrode as the other electrode according to an embodiment ofthe current invention. The configuration shown represents an asymmetricsupercapacitor or a battery.

FIG. 9 is a schematic illustration of a battery and a supercapacitorcombined into a single charge storage device by sharing one electrodeaccording to an embodiment of the current invention. The supercapacitoras well as the battery can be of any type described in FIGS. 1-8.

FIG. 10 is a schematic illustration of energy storage devices fabricatedin a stacked fashion (in serial order) according to an embodiment of thecurrent invention, thus increasing the total voltage output.

FIG. 11 is a schematic illustration of energy storage devices connectedin parallel order according to an embodiment of the current invention,thus increasing the total energy output.

FIG. 12 is a schematic illustration of a battery device connected to asupercapacitor according to an embodiment of the current invention. Thissupercapacitor may be connected either by conventional means or by thenanowire/nanotube networks directly.

FIG. 13 is a schematic illustration of energy storage devices connectedto an electrical circuit according to an embodiment of the currentinvention.

FIG. 14 is a schematic illustration of energy storage devices connectedto an RFID tag according to an embodiment of the current invention.

FIG. 15 is a schematic illustration of energy storage devices connectedto a display according to an embodiment of the current invention.

FIG. 16 is a schematic illustration of energy storage devices connectedto a solar cell according to an embodiment of the current invention.

FIG. 17 is a schematic illustration of a set up to prepare carbonnanotube (CNT) networks by filtration of a CNT suspension according toan embodiment of the current invention.

FIG. 18 is a schematic illustration of a supercapacitor using CNTnetworks as electrodes according to an embodiment of the currentinvention.

FIG. 19 is a SEM (scanning electron microscope) image of a CNT networkused as an electrode in the CNT supercapacitor according to anembodiment of the current invention.

FIG. 20 shows galvanostatic charge/discharge characteristics of a CNTnetwork supercapacitor according to an embodiment of the currentinvention.

FIG. 21 shows a cyclovoltammogram of a CNT network supercapacitoraccording to an embodiment of the current invention.

FIG. 22 shows a cyclovoltammogram of a CNT network, before (inner curve)and after (outer curve) deposition of polyaniline according to anembodiment of the current invention.

FIG. 23 an image of a CNT supercapacitor according to an embodiment ofthe current invention.

FIG. 24 shows galvanostatic charge/discharge of a sprayed CNT basedsupercapacitor using polyvinyl alcohol (PVA)/H₃PO₄ based polymerelectrolyte according to an embodiment of the current invention. Thecurrent applied was 60 μA. The curves in other electrolytes investigatedlook very similar and are left out for clarity.

FIG. 25 shows capacitance of a sprayed CNT based supercapacitor usingvarious liquid electrolytes and a polymer electrolyte according to anembodiment of the current invention. The same CNT network has been usedfor all measurements.

FIG. 26 shows capacitance/Area vs. electrode thickness of a CNT basedsupercapacitor using either a liquid electrolyte or a polymerelectrolyte according to an embodiment of the current invention. PVA Icontains more electrolyte than PVA II.

FIG. 27 shows self-discharge for CNT thin film supercapacitors usingeither 1M H₃PO₄ or a PVA/H₃PO₄-based polymer electrolyte according to anembodiment of the current invention. The t_(0.75)V-values indicate thetime until the voltage has dropped below 0.75 V.

FIG. 28 shows time dependence of voltage during self-discharge for a CNTthin film supercapacitor in liquid 1M H₃PO₄ as electrolyte according toan embodiment of the current invention. The upper curve represents a Vvs. log(t) plot (top axis) and the lower curve a V vs. t^(1/2) plot(bottom axis).

FIG. 29 shows time dependence of voltage during self-discharge for a CNTthin film supercapacitor using PVA/H₃PO₄ as electrolyte according to anembodiment of the current invention. The upper curve represents a V vs.log(t) plot (top axis) and the lower curve a V vs. t^(1/2) plot (bottomaxis).

FIG. 30 shows a Ragone plot including thin film supercapacitors based onCNT networks (black dots) according to an embodiment of the currentinvention.

FIG. 31 shows capacitance/Area before and after spray-coating of anactive material on top of the CNT network according to an embodiment ofthe current invention. The device was completed with a polymerelectrolyte (PVA/H₃PO₄).

FIG. 32 shows internal resistance before and after spray-coating of anactive material on top of the CNT network according to an embodiment ofthe current invention. The device was completed with a polymerelectrolyte (PVA/H₃PO₄).

FIG. 33 shows self discharge of CNT based supercapacitors after 30 mincharging to 1V in a liquid 1M H₃PO₄ solution.

FIG. 34 shows self discharge of CNT based supercapacitors after 30 mincharging to 1V in a polymer electrolyte according to an embodiment ofthe current invention. Note the different time scale.

FIG. 35 shows a device featuring a carbon nanotube network as a chargecollector and a manganese dioxide/carbon nanotube mix as an activeelectrode according to an embodiment of the current invention.

FIG. 36 shows typical discharge curves of devices as depicted in FIG.35. The discharge was measured using various constant discharge currents(0.5 and 1 mA) and MnO₂ to nanotube ratios (40:1 and 80:1 and nonanotubes). Total capacity for these devices are 15 mA h (discharge 1and 2) and 25 mA h (discharge 3) for a discharge to 0.8 V.

FIG. 37 is a schematic illustration of a fully printable and flexiblebattery using carbon nanotubes as a charge collector (protected by PEDOTon the Zinc side), Zinc powder (with electrolyte to create paste) asactive cathode material, MnO₂/carbon nanotube (with electrolyte tocreate paste) mixture as active anode material and polymer electrolyte(electrolyte and PVA mixture) as separator according to an embodiment ofthe current invention.

FIG. 38 shows a comparison of devices made with polymer electrolyteusing Zn powder and Zn sheet electrodes on one hand and using thefiltered CNT films (freestanding CNT paper) as well as the sprayednetworks (airbrush) on the other. Mass efficiency is only 30-40% (ascompared to up to 95% efficiency in the devices using liquidelectrolyte) so far.

FIG. 39 shows resistance of Ag NW networks before and after being coatedwith either PEDOT or CNTs. For comparison, the resistance of the secondcoating only is also shown (center column per data set).

DETAILED DESCRIPTION OF THE INVENTION

In describing embodiments of the present invention illustrated in thedrawings, specific terminology is employed for the sake of clarity.However, the invention is not intended to be limited to the specificterminology so selected. It is to be understood that each specificelement includes all technical equivalents which operate in a similarmanner to accomplish a similar purpose.

All references cited anywhere in this specification are herebyincorporated by reference herein. The term nanostructure is intended tohave a broad meaning to include any structure that has at least onedimension less than about 100 nm and greater than about 1 nm and mayinclude carbon nanotubes, fullerenes, nanowires of various materialsincluding, but not limited to, polymers, nanoparticles, nanoplates andnanorods. A nanostructured network consists of nanostructuredcomponents, including but not limited to polymer nanofibers, inorganicnanoparticles, nanowires, and/or carbon flakes. The term “nanostructurednetwork” is intended to refer to randomly and/or semi-randomly orderednanostructures that are in electrical connection.

The term “active material” has a slightly different meaning in batteriesversus supercapacitors. In a battery, the active material is thematerial which, through a chemical reaction (reduction/oxidationreaction) leads to the observed voltage and provides the electricalcharges. The active material in a supercapacitor is the material thatprovides the high surface area for the double layer capacitance andundergoes the charge transfer reactions (pseudo capacitance).

The term “electrolyte” refers to the material which provides the ionicconductivity between the active materials.

Carbon nanotubes are highly conducting nano-scale wires that can formthin films with low sheet resistance. Such films (G. Gruner et al, J.Mater. Chem. 16, 3533 (2006)) are used for a variety of electronic andsensing (G. Gruner et al, Nano Lett. 6, 2031 (2006)) applications. Thepresent invention can exploit not only the low resistance but also thehigh surface area of the film. FIG. 1 is a schematic illustration of asupercapacitor that incorporates carbon nanotube films. The CNT filmsact as the electrode, in intimate contact with the electrolyte, and alsoserve as the charge collector. In an embodiment of the currentinvention, two CNT films are used, i.e., for both electrodes and chargecollectors. In other embodiments of this invention, other types ofnanostructured films may be used in place of the CNT films, or incombination with such films.

It is an object according to some embodiments of the present inventionto provide a nanostructure-film device for charge storage devices.Specifically, a supercapacitor and a battery device withnanostructure-film as both the electrode and charge collector isprovided according to some embodiments of the current invention.Nanostructure-films include, but are not limited to, carbon nanotubes,graphene flakes and silver nanowires.

In accordance with one embodiment of the present invention, asupercapacitor device and a battery device comprising carbon nanotube(CNT) films is provided. Conducting films composed of randomlydistributed CNTs have been demonstrated as having better electricalproperties than carbon black. Moreover, carbon is one of the mostabundant elements on Earth.

The above embodiment may be a supercapacitor device, comprising at leastone carbon nanotube film on a substrate, a separator and a conductingliquid. In another embodiment, a battery comprising at least one carbonnanotube film on a substrate, a separator and a conducting liquid.Electrical contact pads may also be connected, and thenanostructure-film conducting layer(s) may be patterned (e.g., withphoto lithography and reactive ion etching (RIE)).

A method for fabricating the above-mentioned device embodiments is alsodescribed. The method comprises forming a free standing nanotube film,or depositing a nanostructure-film on a substrate, and subsequentlyforming the devices. The method may further comprise depositing andpatterning the aforementioned electrical contact pads.

EXAMPLE 1

Fabrication of Supercapacitor with Free Standing Carbon Nanotube Films

1.1 Preparation of Carbon Nanotube (CNT) Suspension

1.1.1 CNT material is suspended in an aqueous or organic liquid, using asurfactant if necessary. Ultrasound is used to give a stable andhomogeneous suspension in a reasonable time.

1.1.2. The CNT suspension is filtered through a membrane filter (poresize 0.1-0.01 μm) using a standard flask, a frit as filter holder and amoderate vacuum to suck through the suspension (see FIG. 17).

1.1.3 The filter cake is an entangled network of CNTs. It can be

peeled off from the filter to give a freestanding CNT network which canbe used as an electrode in devices according to some embodiments of thisinvention.

1.2 Preparation of the Capacitor

1.2.1 The freestanding CNT network is glued on a substrate andelectrically contacted in this example. For the substrates, plasticshave been found to work well but glass, fabrics or other material can beused as well. Regarding the glue, double sided scotch tape works as wellas any liquid glue such as epoxy. For the electrical contacts, alligatorclips can be directly clipped to one end of the CNT electrode. Also,silver paste or evaporated/sputtered metals such as gold can be used todraw a conductor path to the CNT electrode. In order to increase theperformance, a conducting polymer was deposited on top of the CNTnetwork (multiple layers).

1.2.2 Two CNT electrodes prepared such as described in 1.1, were wettedwith the electrolyte used. Then, they were stacked on top of each otherusing a porous membrane as separator (see FIG. 18). The electrolyte canbe based on organic or aqueous solutions as well as on ionic liquids.Such electrolytes can be immobilized in a polymer or gel leading toso-called polymer or gel electrolytes. Usually, the use of such polymeror gel electrolytes makes an extra separator unnecessary.

1.3 Test Results

Two methods are used in the following evaluation of CNT electrodes insupercapacitors: cyclic voltammetry and galvanostatic charge/discharge.

Our preliminary results prove that CNT networks can indeed be used aselectrodes in supercapacitors. The results (30-80 F/g) are comparable toother supercapacitors using CNT as electrode material in composites (CNTmaterial mixed with amorphous carbon and optionally a binder). FIG. 19is an SEM image showing the structure of the CNT network and the graphsin FIGS. 20 and 21 are showing a galvanostatic charge/discharge andcyclovoltammograms, respectively. In this device, CNT networks are usedas the sole electrode material. A commercial membrane filter acts asseparator, and polyethylene therephthalate (PET, overhead transparency)is used for both, a substrate and container for the electrolyte, asdepicted in the photograph in FIG. 23.

The larger area within the curve when coated with polyaniline, as shownin FIG. 22, proves that a higher capacitance can be achieved bydeposition of a suitable active material and thus leads to higher energydensity (E=½CV²).

EXAMPLE 2

Supercapacitor with NT Electrode and Charge Collector on a Substrate

2.1 Preparation of the Capacitor

Single wall carbon nanotube (SWCNT) material was dissolved in pure water(1-2 mg/ml) with the aid of a tip sonicator. Using an air brush pistolthe stable suspension was sprayed onto overhead transparencies(polyethylene-therephthalate, PET) which were placed on a heating plateat ˜100° C. During spraying, the water evaporates and the CNTs form anentangled random network on the PET. Afterwards the CNT coated PETsubstrates were used as thin film electrodes without any furthertreatment. The polymer electrolyte was prepared by mixing polyvinylalcohol (PVA) with water (1 g PVA/10 ml H₂O) and subsequent heatingunder stirring to ˜90° C. until the solution becomes clear. Aftercooling down, concentrated phosphoric acid was added (0.8 g) and theviscose solution was stirred thoroughly. Finally, the clear solution wascast into a Petri dish where it was left to let excess water evaporate.Once the polymer electrolyte (H₃PO₄/PVA) is hard, it was cut into piecesserving as both electrolyte and separator in our devices. The H₃PO₄/PVAwas relatively thick (˜1.2 mm) but can be easily decreased by changingthe PVA/Water ratio and by using printing techniques. Liquidelectrolytes of 1M solutions of H₃PO₄, H₂SO₄, and NaCl were prepared forcomparison. For the device assembly, the CNT coated PET substrates weresandwiched together separated by a piece of polymer electrolyte.Galvanostatic charge/discharge experiments have been used to evaluatethe capacitance of such devices using a potentiostat in a 2-electrodeconfiguration.

2.2 Test Results

2.2.1 Capacitance of Thin Film CNT Electrodes in Liquid and PolymerElectrolytes

We compared the H3PO4/PVA-polymer electrolyte with different liquidelectrolytes (1M solutions of H3PO4, H2SO4 and NaCl) in terms of thespecific capacitance of our device. In FIG. 24, a typical galvanostaticcharge/discharge curve of our devices is shown which looks very similarin all electrolytes investigated. The specific capacitances C have beencalculated from such curves using C=I/ΔE, where I is the current appliedand ΔE the voltage range. Also, the internal resistances (IR) of thedevices have been evaluated from the voltage drop at the beginning ofeach branch of the curve.

The evaluated specific capacitances when different electrolytes wereused are summarized in FIG. 25.

When strong acids (H₃PO₄, H₂SO₄) are used the specific capacitance isslightly higher compared to the polymer electrolyte, about 10%. This canbe explained by the fact that both the amount of electrolyte per volumeand the mobility of the ions are somewhat reduced compared to a fullyliquid electrolyte. The internal resistances of our devices weretypically between 500Ω and 600Ω and did not vary significantly with theelectrolytes investigated. The relative high resistance is mainly due tothe thick polymer electrolyte. In general, the specific capacitances ofour thin film devices were about 60 F/g which is comparable toconventional devices using amorphous carbon (a-C) electrodes and liquidelectrolytes.

2.2.2 Optimization of Thickness

The thickness of the thin film CNT electrode is another issue to beoptimized. In general, thicker electrodes provide a higher capacitanceassuming that all material is in contact with the electrolyte. However,in devices, in particular in thin printed devices, the amount of liquidis limited which also limits the penetration depth of the electrolyteinto the porous electrode material. In general, only the part of theelectrode material which is wetted by the electrolyte contributes to thecapacitance leading to an effective thickness (d_(eff)) of theelectrode. Excess electrode material should be avoided as it decreasesthe device performance when correlated to its mass. This makes thin filmelectrodes potentially more effective than thick electrodes, andprinting techniques can take advantage of that as they are limited torather thin films too. In FIG. 26, the dependence of the capacitancefrom the electrode thickness is presented for both a liquid (H₂SO₄) andtwo polymer electrolytes (PVA/H₃PO₄) with different PVA-electrolyteratios.

For thin electrodes (d<1 μm) the capacitance increases linearly with thethickness regardless of the electrolyte used. This confirms the resultspresented in FIG. 25 showing that the capacitance is almost independentfrom the type of electrolyte. For thicker electrodes, however, thesituation is different. Whereas the capacitance still increases linearlyfor the liquid electrolyte, it shows saturation when a polymerelectrolyte is used. This observation leads to the conclusion that thepolymer electrolyte does not entirely penetrate into the porous networkprovided leading to an effective thickness (d_(eff)) which is specificfor the electrode/electrolyte configuration used. In contrast, theliquid electrolyte penetrates through the entire thickness of the CNTelectrodes leading to a maximum wetting of the electrode and maximumperformance. The difference of d_(eff) for liquid and polymerelectrolytes can be explained by a different amount and mobility of theelectrolyte. The disadvantage of a lower absolute capacitance of thinfilm CNT electrodes can easily be compensated by simply printing morelayers. In certain embodiments of the present invention, the networkdensity of thin film electrode layers can be varied to provide differingproperties. For example, a current collector portion of the electrodemay be formed from a relatively denser CNT network (e.g., providinghigher electrical conductivity), while a portion of the electrode thatis in contact with the electrolyte may be formed from a relatively lessdense CNT network (e.g., allowing for better electrolyte penetrationinto the network, and correspondingly higher capacitance). Such anelectrode may be fabricated by, for example, forming a first layer bydepositing a solution (e.g., as described above) having a relativelyhigh concentration of CNTs onto a substrate, and subsequently depositinganother solution having a relatively lower concentration of CNTs overthe first layer to form a second layer (e.g., with a lower CNT networkdensity).

2.2.3 Self-Discharge

Self-discharge refers to the gradual decrease of the voltage across thesupercapacitor when charged and left unconnected. In the absence ofleakage current, self-discharge is mainly due to faradaic reactions ofimpurities, including oxygen species, which can be either activationcontrolled or diffusion controlled.

In practice, both mechanisms are observed and the dominating mechanismdepends often on the voltage applied. Important for practicalconsiderations is the time until the voltage has dropped below 0.75Vassuming that energy cannot conveniently be extracted from a capacitorat voltages less than 0.75V. In FIG. 27, a liquid and a polymerelectrolyte are compared in terms of the self-discharge of a CNT thinfilm supercapacitor.

It is obvious that the polymer electrolyte decreases the self-dischargerate significantly. Its use clearly improved the shelf-life of a CNTthin film supercapacitor which remained operable for about 1 h when putin the charged state. This can be explained by a significant influenceof the polymer electrolyte on the self-discharge mechanism. According toConway et al. the principal mechanisms of self-discharge can bedistinguished by the form of the decline of voltage V in time. If thedominating mechanism is activation controlled, a linear dependence of Vfrom log(t) can be observed. If it is diffusion controlled, the voltagedepends linearly from t^(1/2). This gives an easy diagnostic tool andallows identifying the dominating mechanism simply by plotting thevoltage vs. either log(t) or t^(1/2). In FIGS. 28 and 29, such ananalysis is presented for the self-discharge of CNT-thin filmsupercapacitor using either a liquid (1M H₃PO₄) or a polymer electrolyte(PVA/H₃PO₄).

When a liquid electrolyte is used (FIG. 28) a clear linear decline ofthe voltage cannot be observed for either the V vs. t^(1/2) plot or theV vs. log(t) plot. This indicates that a clear distinction between bothmechanisms is not possible for this case and both activation anddiffusion controlled self-discharge takes place in parallel.

When a polymer electrolyte is used (FIG. 29), however, the situation haschanged. Whereas the V vs. t^(1/2) plot, again, does not give a lineardependence at all a pretty straight line over the entire voltage regimeinvestigated can be observed for the V vs. log(t) plot. This leads tothe conclusion that the self-discharge is dominated by an activationcontrolled mechanism when a polymer electrolyte is used. Obviously, thediffusion controlled mechanism became negligible which can be explainedby the limited mobility of ions as they are trapped in the polymermatrix. Hence, the self-discharge rate is, though not eliminated, atleast slowed down significantly making the use of a polymer electrolytefavorable over a liquid electrolyte.

Energy and Power Density

The energy density vs. power density plot, also called Ragone Plot,allows the comparison of various charge storage devices in terms oftheir performance. In FIG. 30, such a Ragone Plot summarizes theperformance of CNT-based thin film supercapacitor in comparison withfuel cells, batteries and conventional capacitors and supercapacitors.

Even though our devices are not yet optimized in these examples, theirperformance already approaches the higher limit of conventionalsupercapacitors. This is not surprising as CNT networks provide asignificant weight-advantage since a heavy metallic current collector iscompletely unnecessary. Also, the entire CNT network contributes to thedouble layer and no excess material is left maximizing the efficiency ofthin film CNT supercapacitors. The combination of the current collectorand the active layer in a single thin film results in the very goodperformance observed in the Ragone plot. Optimizations towards a higherenergy and power density might lead to a point where the performances ofthin film CNT supercapacitors even exceed the typical range ofconventional devices (black circle). Such optimizations may includeorganic polymer electrolytes for a higher operating voltage and thintop-coatings of other materials for additional pseudo capacitivecontributions.

2.3 Electrolytes

In the embodiment described above one representative electrolyte hasbeen used. Any compound is suitable as electrolyte in supercapacitorswhen its components can be used as mobile ionic species. In order toachieve that requirement, mostly salts are dissolved in a solvent, butsalts liquid at room temperature (ionic liquids) are also possible. Themost common systems to achieve that are listed below.

2.3.1 Aqueous Electrolytes

Usually, inorganic acids, bases and salts are dissolved leading to ionicspecies. For high conductivities, however, solutions of strong acids orbases are usually favored. Examples are given below:

2.3.1.1 Strong Acids: H₂SO₄ (aq), H₃PO₄ (aq), . . .

2.3.1.2 Strong Bases: KOH, NaOH, . . .

2.3.1.3 Moderate pH: Solutions of any compound which dissolves intoionic species, such as salts like NaSO₄, K₂SO₄, LiCl, . . .

2.3.2 Organic Electrolytes, Consisting of a Solvent and an ElectrolyteDissolved in the Solvent. Examples are given below.

2.3.2.1 Solvents:

Ethylene Carbonate (EC), Dimethyl Carbonate (DMC), Propylene Carbonate(PC),

Diethyl Carbonate (DEC), Ethyl Methyl Carbonate (EMC), Dimethylformamide(DMF), Tetrahydrofuran (THF), γButyrolactone, 1,3-Dioxolane (DOL),Methylacetate (MA), Glutaronitrile (GLN), . . .

2.3.2.2 Electrolytes:

Et₄NClO₄, Et₄NBF₄, Et₄NPF₆, Et₄NAsF₆, Et₄NSbF₆, Et₄NNbF₆, Et₄NCF₃SO₃,Et₄NC₄F₉SO₃, Et₄N(CF₃SO₂)₂N, Et₄NBCH₃(C₂H₅)₃, Et₄NB(C₂H₅)₄,Et₄NB(C₄H₉)₄, Et₄NB(C₆H₅)₄, Et₄NB(C₆F₅)₄, LiCF₃(SO₃), LiN(CF₃SO₂)₂,LiClO₄, LiAsF₆, LiBF₄, LiPF₆, . . .

2.3.3 Ionic Liquids

Room temperature ionic liquids are usually quaternary ammonium salts,such as tetralkylammonium [R₄N]⁺, or based on cyclic amines, botharomatic (pyridinium, imidazolium) and saturated (piperidinium,pyrrolidinium). Low-temperature molten salts based on sulfonium [R₃S]⁺as well as phosphonium [R₄P]⁺ cations are also known. Cations may bemodified by incorporating functionalities to carbon atoms of the ring:for example incorporating nitrile to 1-alkyl-3-methylimidazolium. Aswell, anions may be based on cyano groups, such as [Ag(CN)₂]⁻, [C(CN)₃]⁻or [N(CN)₂]⁻. Examples are given below.

2.3.3.1 Imidazolium Based

[MeMeIm]⁺[N(CF₃SO₂)₂]⁻, [MeMeIm]⁺[CF₃SO₃]⁻, [MeMeIm]⁺[CF₃CO₂]⁻,[EtMeIm]⁺[BF₄]⁻, [EtMeIm]⁺[CF₃SO₃]⁻, [EtMeIm]⁺[N(CF₃SO₂)₂]⁻,[EtMeIm]⁺[(CN)₂N]⁻, [BuMeIm]⁺[BF₄]⁻, [BuMeIm]⁺[PF₆]⁻,[BuMeIm]⁺[N(CF₃SO₂)₂]⁻, [PrMeMeIm]⁺[N(CF₃SO₂)₂]⁻,[PrMeMeIm]⁺[C(CF₃SO₂)₃]⁻, . . .

2.3.3.2 Pyrrolidinium Based

[nPrMePyrrol]⁺[N(CF₃SO₂)₂]⁻, [nBuMePyrrol]⁺[N(CF₃SO₂)₂]⁻,[nBuMePyrrol]⁺[N(CF₃SO₂)₂], . . .

2.3.3.3 Tetraalkylammonium Based

[nMe₃BuN]⁺[N(CF₃SO₂)₂]⁻, [nPrMe₃N]⁺[N(CF₃SO₂)₂]⁻,[nOctEt₃N]⁺[N(CF₃SO₂)₂]⁻, [nOctBu₃N]⁺[N(CF₃SO₂)₂]⁻, . . .

2.3.3.4 Pyridinium Based

[BuPyr]⁺[BF₄]⁻, [BuPi]⁺[N(CF₃SO₂)₂]⁻, . . .

2.3.3.5 Piperidinium Based

[MePrPip]⁺[N(CF₃SO₂)₂]⁻, . . .

2.3.3.6 Sulfonium Based

[Et₃S]⁺[N(CF₃SO₂)₂]⁻, [nBu₃S]⁺[N(CF₃SO₂)₂]⁻, . . .

2.3.4 Polymer/Gel Electrolytes

Any of the above mentioned types of electrolytes can be immobilized in apolymer or gel matrix leading to so-called polymer- or gel electrolytes.Here, the electrolyte is trapped in through-connected pores of thematrix resulting in thin solid electrolyte films. Typical polymers forsuch purpose are listed below:

PEO [poly(ethylene oxide)], PAN [poly(acrylonitrile)], PVA [poly(vinylalcohol)], PMMA [poly(methyl methacrylate)], PVDF [poly(vinylidenefluoride)], PVC [poly(vinyl chloride)], MEEP [poly[bis(methoxy ethoxyethoxyphosphazene)], PVS [poly(vinyl sulfone)], PVP [poly(vinylpyrrolidone)], PPO [poly(propylene oxide)], . . .

EXAMPLE 3

Multiple Networks as Electrodes and Charge Collectors

A goal of multiple networks is to take advantage of both the highconductivity of the CNT networks and the high specific capacitance ofthe coating potentially increasing the capacitance of CNT networks. Thesecond material is sprayed on top of the CNT networks. In such amultiple network the CNT network acts rather as the current collectorwhereas the additional coating is the active material. This isfundamentally different from composites where all materials are mixedtogether and potentially interrupting the current paths within the CNTnetwork. The performance of these multiple networks will be discussed interms of capacitance/area and internal resistance.

3.1 Test Results

3.1.1 Capacitance

In FIG. 31, the capacitance/area before and after coating with differentmaterials is presented.

In general, we have observed two trends for our devices:

(a) When using inorganic coatings, here MnO₂ and TiO₂, the capacitancedecreased compared to the not-coated CNT network. This is in contrast tomany publications where high capacitances for these materials have beenreported, explained by additional faradaic reactions. However, suchpseudo capacitive contributions depend strongly on theelectrode/electrolyte combination used. Hence, the electrode/electrolytesystem used here needs to be optimized to take advantage of the pseudocapacitive contributions of such coatings.

(b) When using carbon black and polyaniline coatings, the capacitancehas been increased significantly. This can be explained by a highersurface area and pseudo capacitive contributions (in particular forpolyaniline). The polyaniline coating leads to the highest capacitanceof all materials investigated. But the values are constant overconsecutive cycles since polyaniline tends to degrade when highervoltages are applied. Consequently, the capacitance is decreasing aftera few charge/discharge cycles (FIG. 31, last column). This makes carbonblack the most promising active material for the multiple networkconcept. This way, the high conductivity of the CNT network and the veryhigh surface area of the a-C are combined providing a maximumperformance in a reliable device.

3.1.2 Internal Resistance

In general, the internal resistance determines the power density ofsuper-capacitors. It should be minimized for a maximum current boost. InFIG. 32, the internal resistance before and after coating with differentmaterials is presented.

The internal resistance increased for all coatings except for carbonblack, where the resistance remained almost unchanged. The increasingresistance can be explained by a lower conductivity of the on-coatedmaterial. In addition, the degradation of polyaniline is here obvioustoo as the internal resistance increased with the number ofcharge/discharge cycles. This makes carbon black, again, the favorablecoating for an increased performance of our devices.

3.1.3 Self Discharge

Self-discharge refers to the gradual decrease of the voltage across thesupercapacitor when charged and left unconnected. In the absence ofleakage current, self-discharge is mainly due to faradaic reactions. Itis assumed that energy cannot be extracted at voltages of less than 0.75V. In FIG. 33 and FIG. 34, the self-discharge before and after coatingwith different materials is presented for both a liquid electrolyte (1MH₃PO₄) and a polymer electrolyte (PVA/H₃PO₄).

In general, two conclusions can be drawn from the self-dischargeexperiments:

The use of a polymer electrolyte lowers the self-discharge ratesignificantly compared to a liquid electrolyte (note the different timescales). This can be explained by the limited mobility of ions whentrapped in the polymer matrix indicating that diffusion plays animportant role for discharge mechanism.

Compared to not-coated CNT network, all additional coatings increase theself-discharge rate leading to a decreased shelf-life of the device.When a polymer electrolyte has been used carbon black coatings decreasethe shelf-life the least. In Table 1, the times are listed until thevoltage drops below 0.75V and 0.37V, respectively.

3.2 Electrode Materials for Symmetric and Asymmetric Supercapacitors

In the embodiment described above one electrode material has been used.In general, all materials used in electrochemical devices can also beused in supercapacitors. This includes electrode materials used in

-   -   fuel cells    -   primary and secondary batteries    -   supercapacitors with aqueous, organic, ionic liquid and polymer        electrolytes    -   organic solar cells

For example, carbon (a typical electrode material in supercapacitor,fuel cells) can be combined with metal oxides such as PbO₂, MnO₂ orNi(OOH), typical electrode material in batteries. Also, mixtures ofvarious materials in one electrode can be used as well, e.g. ascomposites, alloys, multiple layers, etc.

Regarding the architecture, a supercapacitor can be either symmetricwhere both electrodes are of the same material, or asymmetric where theelectrodes are of different material.

Examples of typical electrode materials are listed below:

3.2.1 Metals and Metal Oxides:

Zn, Co, Ni, Li, Ru, TiO₂, PbO₂, RuO₂, IrO₂, MnO₂, Fe₃O₄, In₂O₃, WO₃,SnO₂, V₂O₅, Ni(OH)₂, Ni(OOH), LiCoO₂, Li₄Ti₅O₁₂, Ir_(0.3)Mn_(0.7)O₂,etc.

3.2.2 Carbonaceous Materials:

All types of synthetic and natural carbon structures and its derivativessuch as graphite, carbon black, carbon nanotubes, fullerenes, activatedcarbons, carbon cloths, foams, aerogels, etc.

3.2.3 Conducting Polymers:

Polyaniline, polythiophene, polypyrrol, PEDOT, etc.

EXAMPLE 4

Primary Battery with One Nanotube Electrode and Charge Collector

4.1 Device Preparation

In an embodiment of the present invention one can specifically usesingle wall carbon nanotubes (SWCNT) as the nanowire. The followingdevice has been fabricated and tested: a battery device based on theMnO₂-Zinc system as shown in FIG. 36. The charge collector consists of athin film of carbon nanotubes created by a filtration process asdescribed in the previous embodiments. A similar SWCNT solution to theone used in this process would also be suitable for other printingprocesses such as ink jet printing (see Example 2). The active materialof the cathode (MnO₂) features very poor electrical conductivityresulting in very poor discharge behavior. This problem has beenresolved here by mixing the MnO₂ powder with carbon nanotubes(MnO₂:SWCNT=1:20 (weight:weight) in this case) resulting in a highconductivity and providing conducting paths to the charge collector forthe electrons produced in the chemical reaction. A standard electrolyte(NH₄Cl:ZnCl₂:H₂O=26%:8.8%:65.2% weight) and anode (Zinc foil) have beenused in this device. Both the separator and the cathode mix have beensoaked in electrolyte.

4.2 Test and Results

The discharge curve of the device (FIG. 37) shows a distinguishedplateau with a total energy capacity of at least 50 mAh.

EXAMPLE 5

Battery with Two Nanotube Electrodes/Charge Collectors

5.1 Device Preparation

By adding both printable anode and electrolyte, a new fully printablearchitecture results as shown in FIG. 38. The cathode in the devicedescribed earlier consisted of a sheet of zinc. In order to fabricate aprintable cathode, zinc powder seems to be the choice of material. Theultimate goal of the battery design is to use as much of the activematerial as possible in the chemical reaction powering the battery.Thus, the electrode design should not rely on the active material itselffor current collection, as the progression of the chemical reactionwould make current collection impossible at some point. In conventionalbatteries, the current collection is guaranteed by having a substantialsurplus of zinc (i.e. the zinc can in AA batteries has more mass thancan be used by the battery reaction). To minimize the excess weight, wehave decided to replace the current collector with a carbon nanotubenetwork in analogy to the cathode design. However, there is one problemwith this concept: mixing carbon and zinc results in a chemicalreaction, consuming the active anode material (Zn) and thusself-discharging the battery. The solution is to physically separate thezinc form the carbon. We have managed this by applying a thin layer ofconductive polymer consisting of Baytron P (aqueous dispersion ofpoly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT/PSS)))mixed with 5% Dimethyl sulfoxide (DMSO) for enhanced conductivity. Whilethe polymer layer increases the resistance of the current collector byadding an additional layer which is less conductive than the carbonnanotube film by a factor of about 2, the resulting resistance is stillsignificantly better than a pure polymer electrode could manage.Addition of electrolyte to the zinc powder produces a zinc paste thatcan be applied to the polymer-protected carbon nanotube currentcollector, resulting in a three layer anode.

The final step towards a fully printable battery is a polymerelectrolyte (PEL).

A mixture of 14% (wt) Polyvinyl alcohol (PVA) and 86% (wt) electrolyte(consisting of 16.25 g NH₄Cl and 5.5 g ZnCl₂ dissolved in 100 ml water)was heated up to approximately 90° C. in a water bath while beingstirred continuously. Subsequently the viscose polymer-electrolytemixture can be applied to the electrodes. The polymer electrolytesolidifies into a flexible polymer layer upon cooling down.

5.2 Test and Results

The combination of theses layers results in a flexible battery that canbe bent. The voltage of the battery stays approximately constant uponbending to different radii confirmed with a multimeter. The battery hasbeen bent around cylinders with diameters of 91 mm, 56 mm, 27.5 mm, and12.7 mm, in both directions (denoted by positive and negative diametersin table 2) including flat conformations (denoted by d=∞) after everyother measurement. As can be seen in Table 2, the voltage fluctuated byup to 4% during the experiment, but without showing an obvious trendtowards decreasing voltage. The fluctuations can probably be attributedto changes in the pressure holding together the active materials, theelectrolyte and in consequential changes in their contact areas betweeneach other and the CNT nanotube network. In order to verify that theabove described bending of the batteries did not damage the batteries,the voltage was checked again the next day. The tested devices stillexhibited the expected voltage. No long term investigations have beenmade to ensure the self discharge of the batteries is not affected bythe bending. However, one would expect the batteries to be dischargedafter a short time if the bending seriously damaged the devices (e.g.produced a short circuit between the electrodes). Other, smallerpossible disadvantages to the lifetime of the devices caused by thebending, e.g. through dislocation of parts of the active material,should be easily countered by adding a binder to the active materialmix.

In order to evaluate the different components, measurements of devicesmade from Zinc sheet/PEL/MnO₂ have been compared to devices made fromZinc powder/PEL/MnO₂ in addition to the measurements shown earlier (Zincsheet/liquid electrolyte/MnO₂). As can be seen in FIG. 39, the batteriesincluding PEL are not able to exhaust as much of the active material(MnO₂) as the ones with liquid electrolyte. There seems to be nodifference between devices using Zn sheet and devices using the flexibleZn powder electrode concerning the efficiency. This indicates that thereason for this lies in the use of the polymer electrolyte. Use of adifferent polymer, electrolyte, mixing ratio and/or geometry mightresolve this problem. The efficiency reached in our devices has notexceeded 40% so far. This is a notable contrast to the liquidelectrolyte devices reported earlier, which reached values of up to 95%.The measured voltage in these devices is somewhat lower, which might becaused by the additional resistance introduced due to the CNT networkresistance, the polymer layer resistance and contact resistances betweenthose layers, the zinc powder and the PEL.

5.3 Materials for Alternative Batteries

In the example described above, one electrode material was used.

Other combinations of active materials/electrolytes could be used withthe design described here. Some of the most common combinations, withoutlimitation, are listed below:

5.3.1 Primary Batteries

5.3.1.1 Zinc-Carbon Batteries:

Active materials: Zinc (Zn) and manganese dioxide (MnO₂).

Electrolyte: Instead of the described electrolyte (NH₄Cl, ZnCl₂ andwater), one could use either ZnCl₂ in water (without NH₄Cl) or anaqueous solution of KOH (alkaline battery).

5.3.1.2 Zinc/Air Batteries:

Active materials: Zinc (Zn) and oxygen (O₂, air).

Electrolyte: KOH (aqueous solution).

5.3.1.3 Mg/MnO₂ Batteries:

Active materials: Magnesium (Mg) and manganese dioxide (MnO₂).

Electrolyte: Aqueous solution of MgBr₂ and Mg(ClO₄).

5.3.1.4 Zn/Hgo Batteries

Active materials: Zinc (Zn) and mercury oxide (HgO).

Electrolyte: KOH or NaOH (aqueous solutions).

5.3.1.5 Aluminum Batteries:

Active materials: Aluminum (Al) and oxygen (O₂, air).

Electrolyte: several possible electrolytes, including aqueous KOH.

5.3.1.6 Cd/Hgo Batteries:

Active materials: Cadmium (Cd) and mercury oxide (HgO).

Electrolyte: KOH (aqueous solution).

5.3.1.7 Zn/Ag₂O Batteries:

Active materials: Zinc (Zn) and silver oxide (Ag₂O or AgO).

Electrolyte: KOH or NaOH (aqueous solutions).

5.3.1.8 Lithium Batteries:

Active materials: Lithium (Li) and sulfur dioxide (SO₂), manganesedioxide (MnO₂), FeS₂.

Electrolyte: Organic solvent, salt solution or SOCl₂ with AlCl₄respectively.

5.3.1.9 Solid State Batteries:

Active materials: Lithium (Li), I₂(P2VP).

Electrolyte: solid

5.3.2 Secondary Batteries

5.3.2.1 Lithium Ion Batteries:

Active materials: Lithium-metal-oxides (such as LiCoO₂,Li_(1-x)CO_(1-y)M_(y)O₂ etc.) or phosphate based (e.g. LiFePO₄,Li₃V2(PO₃)₃) and usually carbon (sometimes nitrides, sulfides,phosphides or oxides such as CuO)

Electrolyte: lithium-salt electrolytes (such as LiPF₆, LiBF₄, or LiClO₄)in organic solvents (aqueous or as polymer electrolytes).

5.3.2.2 Silver-Zinc Batteries:

Active materials: Zinc (Zn) and silver oxide (AgO).

Electrolyte: KOH (aqueous solution).

5.3.2.3 Zinc-Carbon:

Active materials: Zinc (Zn) and manganese dioxide (MnO₂).

Electrolyte: KOH (aqueous solution).

5.3.2.4 Lead-Acid Batteries:

Active materials: Lead (Pb) and lead dioxide (PbO₂).

Electrolyte: H₂SO₄ (aqueous solution).

5.3.2.5 Nickel-Cadmium Batteries:

Active materials: Cadmium (Cd) and NiOOH.

Electrolyte: KOH (aqueous solution).

5.3.2.6 Nickel-Iron Battery:

Active materials: Iron (Fe) and NiOOH.

Electrolyte: KOH (aqueous solution).

5.3.2.7 Nickel-Metal Hydride Battery:

Active materials: Metal hydride (MH) and NiOOH.

Electrolyte: KOH (aqueous solution).

5.3.2.8 Nickel-Zinc Battery:

Active materials: Zinc (Zn) and NiOOH.

Electrolyte: KOH (aqueous solution).

5.3.2.9 Nickel-Hydrogen Battery:

Active materials: Hydrogen (H₂) and NiOOH

Electrolyte: KOH (aqueous solution).

5.3.2.10 Polymers:

Active materials: Organic functional polymers.

EXAMPLE 6

Multi Layered Nanowire Networks

The following two systems of dual layer nanowire networks have beenproduced:

1. Carbon nanotube network on top of a silver nanowire network.

2. PEDOT layer on top of a silver nanowire network.

The Ag nanowire networks have been fabricated using an airbrush to applythe nanowire suspension onto a PET substrate. Since the Ag nanowires arecovered by a polymer, the network should be annealed to approximately200

° C. in order to achieve good conductivity.

After measuring the resistance [Ω/cm] of the Ag nanowire network using amultimeter and copper clips, a second layer (CNT or PEDOT) were appliedon top of the silver. For comparison, the carbon nanotubes andconducting polymer was also applied to a fresh substrate (PET) at thesame time (same thickness, same preparation etc.). The new resistance[Ω/cm] of the resulting double layer was then determined in the same wayas above. FIG. 39 shows the resulting resistances. The contactresistance between the two networks is obviously smaller than theresistance of the less conducting network (CNT, PEDOT). In the case ofAg NW network/CNT network, the resulting resistance is about the same(maybe even less, but not significant because of the error bars, notshown in the chart) as the starting resistance. As an additionalpositive effect of the second layer, the resulting structures were moreresistant against physical damage than the original Ag nanowire films.

From the foregoing it can be seen that the present invention can beembodied in various ways, including, but not limited to, the following:

1. An energy storage device, comprising: an electrode; a chargecollector; and an electrolyte; wherein the electrode is in physicalcontact with the electrolyte and the charge collector; and wherein atleast one of the electrode and the charge collector comprises ananostructured network.

2. The energy storage device of embodiment 1, wherein the electrodecomprises a first nanostructured network; and wherein the chargecollector comprises a second nanostructured network.

3. The energy storage device of embodiment 2, wherein the energy storagedevice is flexible.

4. The energy storage device of embodiment 3, wherein at least one ofthe first nanostructured network and the second nanostructured networkhas a conductivity of at least about 500 S/cm.

5. The energy storage device of embodiment 4, wherein at least one ofthe first nanostructured network and the second nanostructured networkhas a conductivity of at least about 100 S/cm and less than about 10,000S/cm.

6. The energy storage device of embodiment 5, wherein at least one ofthe first nanostructured network and the second nanostructured networkcomprises a plurality of electrically conducting nanowires.

7. The energy storage device of embodiment 6, wherein at least one ofthe first nanostructured network and the second nanostructured networkcomprises a plurality of carbon nanotubes.

8. The energy storage device of embodiment 5, wherein the firstnanostructured network and the second nanostructured network bothcomprise networks of electrically conducting nanowires.

9. The energy storage device of embodiment 8, wherein the firstnanostructured network has a different nanostructure density than thesecond nanostructured network.

10. The energy storage device of embodiment 9, wherein the firstnanostructured network has a lower nanostructure density than the secondnanostructured network.

11. An energy storage device, comprising: an electrode; a chargecollector; an electrical contact pad; and an electrolyte; wherein theelectrode is in physical contact with the electrolyte and the chargecollector; wherein the electrical contact pad is in physical contactwith the charge collector; and wherein the electrode and the chargecollector comprise a nanostructured network.

12. The energy storage device of embodiment 11, wherein thenanostructured network comprises electrically conducting nanowires andhas a conductivity of at least about 100 S/cm and less than about 10,000S/cm.

13. The energy storage device of embodiment 12, wherein the electrolytepenetrates into a first layer of the nanostructured network, and whereinthe electrolyte does not penetrate into a second layer of thenanostructured network.

14. The energy storage device of embodiment 13, wherein the second layerof the nanostructured network has a higher nanostructure density thanthe first layer of the nanostructured network.

15. The energy storage device of embodiment 14, wherein thenanostructured network comprises a network of carbon nanotubes, andwherein the energy storage device is flexible.

16. An energy storage device, comprising: a first electrode and a secondelectrode; a first charge collector and a second charge collector; andan electrolyte, wherein the first electrode is in physical andelectrical contact with the first charge collector; wherein the secondelectrode is in physical and electrical contact with the second chargecollector; wherein the electrolyte is in physical contact with the firstelectrode and the second electrode; and wherein the first electrode andthe first charge collector comprise a first nanostructured network.

17. The energy storage device of embodiment 16, further comprising: afirst electrical contact pad and a second electrical contact pad;wherein the first electrical contact pad is in physical and electricalcontact with the first charge collector; wherein the second electricalcontact pad is in physical end electrical contact with the second chargecollector; and wherein the first electrical contact pad and the secondelectrical contact pad are connected to an external circuit.

18. The electrical storage device of embodiment 17, wherein the firstnanostructured network comprises a network of electrically conductivenanowires.

19. The energy storage device of embodiment 18, wherein the secondelectrode and the second charge collector comprise a secondnanostructured network, and wherein at least one of the firstnanostructured network and the second nanostructured network comprises anetwork of carbon nanotubes.

20. The energy storage device of embodiment 19, wherein at least one ofthe first nanostructured network and the second nanostructured networkfurther comprises a plurality of metal oxide nanostructures.

The current invention is not limited to the specific embodiments of theinvention illustrated herein by way of example, but is defined by theclaims. One of ordinary skill in the art would recognize that variousmodifications and alternatives to the examples discussed herein arepossible without departing from the scope and general concepts of thisinvention.

TABLE 1 Self-discharge rates for CNT based supercapacitors after 30 mincharging to 1 V in either a polymer electrolyte (2nd and 3rd column) ora liquid electrolyte (last two columns). Note that the time scales aredifferent. Electrolyte PVA/H3PO4 1M H3PO4 t (0.75) t (0.37) t (0.75) t(0.37) Electrode material [min] [h] [min] [min] CNT only 60 58 12 192CNT/MnO2 18 3 4.5 11 CNT/TiO2 33 26 1.5 12 CNT/Carbon Black 40 42 2 9CNT/Polyaniline 10 10 0.5 1

TABLE 2 Bending measurement. While bending the device to differentcurvatures, both the terminal voltage U and the dc resistance of thebattery R (to detect possible short circuits) have been measured. Theterminal voltage seems to stay stable at approximately 1.3 Volts and noshorts (R ≠ ∞) have been detected during the experiment. d [mm]curvature [mm⁻¹] R [Ω] U [V] ∞ 0 ∞ 1.357 91 0.01 ∞ 1.365 56 0.02 ∞ 1.310∞ 0 ∞ 1.329 27.5 0.04 ∞ 1.334 12.7 0.08 ∞ 1.329 ∞ 0 ∞ 1.331 −91 −0.01 ∞1.337 −56 −0.02 ∞ 1.333 ∞ 0 ∞ 1.315 −27.5 −0.04 ∞ 1.338 −12.7 −0.08 ∞1.346 ∞ 0 ∞ 1.301

We claim:
 1. A device for use as an electrode or charge collector in anenergy storage device, the device, comprising: (a) an electricalcontact; (b) a first nanostructured network of a nanoscale electricallyconducting material connected to the electrical contact; and (c) asecond nanostructured network of an electrochemically active material incontact with the first nanostructured network; (d) wherein said firstnanostructured network comprises: a layer of a first electricallyconducting material; and a layer of a second electrically conductingmaterial in contact the layer of a first electrically conductingmaterial; wherein said first and second electrically conductingmaterials have different densities.
 2. A device as recited in claim 1,wherein said nanoscale electrically conducting material is selected fromthe group of conductors consisting of carbon nanotubes, fullerenes,graphene, graphene flakes, metal nanowires, Ag nanowires, a mixture oftwo carbonaceous materials having different electrical conductingproperties, a mixture of a carbonaceous material and a metal material,and conductive polymer nanowires.
 3. A device as recited in claim 1,wherein said electrochemically active material is selected from thegroup of materials consisting of polyaniline, polythiophene, polypyrrol,PEDOT and carbon black.
 4. A device as recited in claim 1, wherein saidelectrochemically active material is selected from the group ofmaterials consisting of Zn, Co, Ni, Li, Ru, TiO₂, PbO₂, RuO₂, IrO₂,MnO₂, Fe₃O₄, In₂O₃, WO₃, SnO₂, V₂O₅, Ni(OH)₂, Ni(OOH), LiCoO₂,Li₄Ti₅O₁₂, and Ir_(0.3)Mn_(0.7)O₂.
 5. A device as recited in claim 1:wherein said first layer of electrically conducting material comprisescarbon nanotubes; and wherein said second layer of electricallyconducting material comprises carbon black.
 6. A device as recited inclaim 1: wherein said first layer of electrically conducting materialcomprises carbon nanotubes having a first density; and wherein saidsecond layer of electrically conducting material comprises carbonnanotubes having a second density.
 7. A device as recited in claim 1:wherein said first layer of electrically conducting material comprisesmetal nanowires; and wherein said second layer of electrical conductingmaterial comprises carbon nanotubes.
 8. A device as recited in claim 1:wherein said first layer of electrically conducting material comprises acarbonaceous material having a first electrical conducting property; andwherein said second layer of electrically conducting material comprisesa carbonaceous material having a second electrical conducting propertydifferent than said first electrical conducting property.
 9. A device asrecited in claim 1, further comprising a conductive polymer protectivelayer disposed between the first nanostructured network and the secondnanostructured network.
 10. A device as recited in claim 1, wherein thefirst nanostructured network has a thickness that is greater or lessthan the second nanostructured network.
 11. A device as recited in claim1, wherein the first nanostructured network interpenetrates the secondnanostructured network to form a composite structure.
 12. An energystorage device, comprising: (a) a first electrode; (b) a secondelectrode; and (c) an electrolyte between the first and secondelectrodes; (d) wherein at least one said electrode comprises: anelectrical contact; a first nanostructured network of a nanoscaleelectrically conducting material connected to the electrical contact;and a second nanostructured network of an electrochemically activematerial in contact with the first nanostructured network; (e) whereinsaid first nanostructured network comprises: a layer of a firstelectrically conducting material; and a layer of a second electricallyconducting material in contact the layer of a first electricallyconducting material; wherein said first and second electricallyconducting materials have different densities.
 13. The energy storagedevice of claim 12, wherein said nanoscale electrically conductingmaterial is selected from the group of conductors consisting of carbonnanotubes, fullerenes, graphene, graphene flakes, metal nanowires, Agnanowires, a mixture of two carbonaceous materials having differentelectrical conducting properties, a mixture of a carbonaceous materialand a metal material, and conductive polymer nanowires.
 14. The energystorage device of claim 12, wherein said electrochemically activematerial is selected from the group of materials consisting ofpolyaniline, polythiophene, polypyrrol, PEDOT and carbon black.
 15. Theenergy storage device of claim 12, wherein said electrochemically activematerial is selected from the group of materials consisting of Zn, Co,Ni, Li, Ru, TiO₂, PbO₂, RuO₂, IrO₂, MnO₂, Fe₃O₄, In₂O₃, WO₃, SnO₂, V₂O₅,Ni(OH)₂, Ni(OOH), LiCoO₂, Li₄Ti₅O₁₂, and Ir_(0.3)Mn_(0.7)O₂.
 16. Theenergy storage device of claim 12: wherein said first layer ofelectrically conducting material comprises carbon nanotubes; and whereinsaid second layer of electrically conducting material comprises carbonblack.
 17. The energy storage device of claim 12: wherein said firstlayer of electrically conducting material comprises carbon nanotubeshaving a first density; and wherein said second layer of electricallyconducting material comprises carbon nanotubes having a second density.18. The energy storage device of claim 12: wherein said first layer ofelectrically conducting material comprises metal nanowires; and whereinsaid second layer of electrical conducting material comprises carbonnanotubes.
 19. The energy storage device of claim 12: wherein said firstlayer of electrically conducting material comprises a carbonaceousmaterial having a first electrical conducting property; and wherein saidsecond layer of electrically conducting material comprises acarbonaceous material having a second electrical conducting propertydifferent than said first electrical conducting property.
 20. The energystorage device of claim 12, further comprising a conductive polymerprotective layer disposed between the first nanostructured network andthe second nanostructured network.
 21. The energy storage device ofclaim 12, wherein the first nanostructured network has a thickness thatis greater or less than the second nanostructured network.
 22. Theenergy storage device of claim 12, wherein the first nanostructurednetwork interpenetrates the second nanostructured network to form acomposite structure.